Conventionally, in the case of a metal product, such as an engine part, which is subjected to a high-temperature gas, a heat insulating layer is formed on a surface of a metal base material of the metal product to suppress heat transfer from the high-temperature gas to the metal product, that is, to suppress a cooling loss of the metal product. One known example is that a heat insulating layer made of an inorganic oxide (such as zirconia) or an organic material containing hollow particles is formed on a top surface of a piston main body that defines a combustion chamber of an engine.
A squish area may be formed at a gap portion between the top surface of the piston main body that defines the combustion chamber and a lower surface of a cylinder head. In a case where the heat insulating layer is provided on a surface (squish area surface), where the squish area is formed, of the top surface of the piston main body, the heat insulating layer becomes high in temperature, and therefore, the squish area surface itself becomes high in temperature. On this account, when a high temperature and high pressure end gas (an uncombusted air-fuel mixture existing far from a spark plug) flows to the squish area in a combustion process, heat radiation from the end gas to the squish area surface is inhibited by the high temperature squish area surface, and this may cause knocking. Then, cracks are generated on the heat insulating layer formed on the squish area surface, and this causes damages and peeling of the heat insulating layer. Thus, a heat insulation property of the heat insulating layer is lost.
Some references (see PTL 1, for example) describe an internal combustion engine in which the heat insulating layer is not formed on the squish area surface of the top surface of the piston main body, and the heat insulating layer is formed only on the other portion of the top surface of the piston main body.
According to the internal combustion engine of PTL 1, since the heat insulating layer is not formed on the squish area surface, the heat radiation from the end gas to the squish area surface is promoted, and the generation of the knocking is suppressed.